J/AJ/148/27 Photometry in globular clusters. I. NGC 1851 (Cummings+, 2014) ================================================================================ Uncovering multiple populations with Washington photometry. I. The globular cluster NGC 1851. Cummings J.D., Geisler D., Villanova S., Carraro G. =2014AJ....148...27C ================================================================================ ADC_Keywords: Clusters, globular ; Photometry, CMT1T2V Keywords: globular clusters: individual: NGC 1851 - stars: imaging - Hertzsprung-Russell and C-M diagrams - stars: abundances Abstract: The analysis of multiple populations (MPs) in globular clusters (GCs) has become a forefront area of research in astronomy. Multiple red giant branches (RGBs), subgiant branches (SGBs), and even main sequences (MSs) have now been observed photometrically in many GCs, while broad abundance distributions of certain elements have been detected spectroscopically in most, if not all, GCs. UV photometry has been crucial in discovering and analyzing these MPs, but the Johnson U and the Stromgren and Sloan u filters that have generally been used are relatively inefficient and very sensitive to reddening and atmospheric extinction. In contrast, the Washington C filter is much broader and redder than these competing UV filters, making it far more efficient at detecting MPs and much less sensitive to reddening and extinction. Here, we investigate the use of the Washington system to uncover MPs using only a 1 m telescope. Our analysis of the well-studied GC NGC 1851 finds that the C filter is both very efficient and effective at detecting its previously discovered MPs in the RGB and SGB. Remarkably, we have also detected an intrinsically broad MS best characterized by two distinct but heavily overlapping populations that cannot be explained by binaries, field stars, or photometric errors. The MS distribution is in very good agreement with that seen on the RGB, with ~30% of the stars belonging to the second population. There is also evidence for two sequences in the red horizontal branch, but this appears to be unrelated to the MPs in this cluster. Neither of these latter phenomena have been observed previously in this cluster. The redder MS stars are also more centrally concentrated than the blue MS. This is the first time MPs in an MS have been discovered from the ground, and using only a 1 m telescope. The Washington system thus proves to be a very powerful tool for investigating MPs, and holds particular promise for extragalactic objects where photons are limited. Description: Our observations of NGC 1851 were performed at the SWOPE 1m telescope at Las Campanas Observatory. Both the R and T2 observations were performed during gray time on 2011-10-21 with one short C image taken that night before the Moon rose, and the remainder of the C observations were performed before the Moon rose on 2011-10-25. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table2.dat 95 18460 The Washington photometric catalog of NGC 1851 -------------------------------------------------------------------------------- See also: VII/202 : Globular Clusters in the Milky Way (Harris, 1997) J/A+A/539/A19 : Horizontal branch stars in NGC 1851 (Gratton+, 2012) J/A+A/533/A69 : Spectroscopy of 124 RGB stars in NGC 1851 (Carretta+, 2011) J/AJ/122/2569 : VI photometry of NGC 288, 362 and 1851 (Bellazzini+, 2001) J/PASP/104/1063 : BV Color-Magnitude Diagram for NGC 1851 (Walker 1992) Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 5 I5 --- Name [2/18461] Source identifier <[CGV2014] NNNNN> in Simbad 7- 13 F7.2 pix Xpos X pixel coordinate (scale=0.435arcsec/pix) 15- 21 F7.2 pix Ypos Y pixel coordinate (scale=0.435arcsec/pix) 23- 29 F7.3 mag Cmag ?=-99.999 Final Washington C band magnitude 31- 37 F7.3 mag e_Cmag ?=-99.999 Error in Cmag 39- 45 F7.3 mag s_Cmag ?=-99.999 Photometric dispersion in Cmag 47- 52 F6.3 mag T1mag Final Washington T1 band magnitude 54- 58 F5.3 mag e_T1mag Error in T1mag 60- 64 F5.3 mag s_T1mag ?=-99.999 Photometric dispersion in T2mag 66- 72 F7.3 mag T2mag ?=-99.999 Final Washington T2 band magnitude 74- 80 F7.3 mag e_T2mag ?=-99.999 Error in T2mag 82- 88 F7.3 mag s_T2mag ?=-99.999 Photometric dispersion in T2mag 90- 91 I2 --- o_Cmag [0/11] Number of independent observations for Cmag 93 I1 --- o_T1mag [1/4] Number of independent observations for T1mag 95 I1 --- o_T2mag [0/4] Number of independent observations for T2mag -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 13-Oct-2014